Interaction diagrams are models that describe how groups of objects collaborate in some behavior.
Typically, an interaction diagram captures the behavior of a single use case. The diagram shows a number of example objects and the messages that are passed between these objects within the use case.
I'll illustrate the approach with a simple use case that exhibits the following behavior.
The Order Entry window sends a "prepare" message to an Order.
The Order then sends "prepare" to each Order Line on the Order.
Each Order Line checks the given Stock Item.
If this check returns "true," the Order Line removes the appropriate quantity of Stock Item from stock and creates a delivery item.
If the Stock Item has fallen below the reorder level, that Stock Item requests a reorder.
There are two kinds of interaction diagrams: sequence diagrams and collaboration diagrams.